In the case of piston units for radial piston machines, various approaches are known.
Document DE 40 37 455 C1 presents a radial piston machine having a plurality of cylinders arranged in a stellate manner in a rotor. Each cylinder is assigned a piston unit. The piston unit has a stepped piston which is supported radially to the outside via a respective roller on a stroke cam of the radial piston machine. Since the stroke cam is formed as a stator and has an undulating shape, the stepped piston performs an oscillating stroke movement when the rotor rotates. The rollers are of circular cylindrical form over their entire length.
A disadvantage of such piston units is the friction that arises between the stepped piston and the associated roller. Furthermore, a high Hertzian stress generated in particular at the end portions of the circular cylindrical rollers is disadvantageous.
To reduce the Hertzian stress, it is known according to the prior art to provide continuous diameter reductions, that is to say crowned formations, at the end portions of the rollers.
Document DE 27 31 474 A1 presents piston units in which a pressure relief means is provided between a piston and a roller. Said pressure relief means has a shallow depression arranged on the piston in a region of contact with the roller. Said shallow depression is charged with working pressure from the associated cylinder via a duct which extends through the piston.
The length of a depression of a pressure relief means of said type is restricted in the case of crowned rollers because the depression can extend only along the circular cylindrical portion of the roller.
Accordingly, document DE 27 31 474 A1 presents a piston unit with a pressure relief means and with a hollow cylindrical roller which has a relatively thin and weak wall. Here, the interior of the roller is charged with working pressure of the associated cylinder. In this way, the Hertzian stress of the weakened roller caused by contact with the stroke cam is reduced.
A disadvantage of the latter piston unit is the outlay in terms of apparatus for the reduction of the Hertzian stress.